1. Field of Invention
The invention relates generally to method and apparatus for lifting (e.g., dechucking) a substrate from a substrate support in a semiconductor processing chamber.
2. Background of the Invention
Liquid crystal displays or flat panels are commonly used for active matrix displays such as computer and television monitors. Generally, flat panels comprise two glass plates having a layer of liquid crystal material sandwiched therebetween. At least one of the glass plates includes at least one conductive film disposed thereon that is coupled to a power source. Power, supplied to the conductive film from the power supply, changes the orientation of the crystal material, creating a patterned display. One fabrication process frequently used to produce flat panels is plasma enhanced chemical vapor deposition (PECVD).
Plasma enhanced chemical vapor deposition is generally employed to deposit thin films on a substrate such as a flat panel or semiconductor wafer. Plasma enhanced chemical vapor deposition is generally accomplished by introducing a precursor gas into a vacuum chamber that contains a substrate. The precursor gas is typically directed through a distribution plate situated near the top of the chamber. The precursor gas in the chamber is energized (e.g., excited) into a plasma by applying RF power to the chamber from one or more RF sources coupled to the chamber. The excited gas reacts to form a layer of material on a surface of the substrate that is positioned on a heated substrate support. A shadow frame and optionally a purge gas is routed through holes in the support to the edge of the substrate to prevent deposition at the substrate""s edge that may cause the substrate to adhere to the support. Volatile by-products produced during the reaction are pumped from the chamber through an exhaust system.
After the deposition process, the substrate is lifted (e.g., dechucked) from the substrate support by a plurality of lift pins disposed through the substrate support. The lift pins are actuated upwards to contact the backside of the substrate and raise the substrate above the substrate support""s upper surface. From this position, a substrate transfer mechanism, typically a blade coupled to a robot, may be positioned between the substrate and the substrate support. The lift pins are retracted, leaving the substrate on the transfer mechanism that is now free to remove the substrate from the processing chamber.
During the deposition process, the substrate may become residually attracted to the substrate support. For example during plasma processing, charge generally accumulates on the substrate due to the difference in mobility between ions and electrons within the plasma. Typically, the more active and mobile electrons reach the substrate in greater numbers than the ions, resulting in a net charge accumulation. The accumulated electrons on the substrate""s surface facing the plasma results in a corresponding accumulation of positive charge on the substrate""s backside, creating a static attraction between the substrate and the substrate support.
As the lift pins begin to lift the substrate from the substrate support, the center region of the substrate (i.e., the region between the lift pins) remains attracted to the substrate support, causing the substrate to bow. As the lift pins continue to move farther away from the substrate support, the force separating the substrate from the substrate support overcomes the static attraction allowing the substrate to regain a substantially planar or slightly bowed form as the substrate is supported by the lift pins in a spaced-apart relation to the substrate support.
However, if the static attraction is great enough during the dechucking process, the substrate bows excessively before the substrate becomes completely separated from the substrate support. The excessive bowing of the substrate may result in cracking, breakage or other damage to the substrate and/or material layers or devices disposed on the substrate. Additionally, excessive bowing of the substrate may cause the lift pins to slide slightly on the backside of the substrate, scratching the substrate and increasing the probability of particulate generation that may contaminate the substrate or other substrates subsequently processed in the chamber.
The static attraction may be dissipated by introducing plasma formed from an inactive gas in the chamber during the dechucking process. The plasma redistributes the charges across the substrate, minimizing the static attraction between the substrate and the substrate support. This process for reducing the static attraction was disclosed in U.S. Pat. No. 5,380,566, issued Jun. 10, 1995, by Robertson et al. Although as little as two seconds of exposure to the plasma formed from the inactive gas may be enough to dissipate static attraction forces and allow the substrate to be dechucked without damage, it is desirable to process the substrate with as few steps as possible and to minimize the consumables such as gases used during processing.
Therefore, there is a need for a substrate support that prevents excessive bowing of the substrate during dechucking.
One aspect of the invention generally provides a substrate support assembly for supporting a substrate during processing. In one embodiment, a support assembly includes a substrate support having a support surface, a first set of lift pins and a second set of lift pins movably disposed through the substrate support. The first set of lift pins and the second set of lift pins project from the support surface when the pins are in an actuated position. When in the actuated position, the first set of lift pins project a greater distance from the support surface than the second set of lift pins. In another embodiment, a support assembly includes a lift plate that contacts the first set of lift pins before contacting the second set of lift pins.
In another aspect of the invention, a method for dechucking a substrate from a substrate support is provided. In one embodiment, the method includes the steps of projecting a first set of lift pins a first distance above a surface of a substrate support to lift a substrate, and projecting a second set of lift pins a second distance above the surface of the substrate support that is less than the first distance. In another embodiment, the second set of lift pins contact the substrate if the substrate bows more than a predetermined amount.